The present invention relates to a process for the degradation of highly toxic organic compounds, in particular polychlorinated dibenzodioxins (PCDD) and dibenzofurans (PCDF), contained in gases containing no SO.sub.2 wherein the highly toxic compounds are removed from the gas by oxidation. The gases may in particular be waste gases from chemical processes and waste gases from refuse incinerators, wherein the gases may additionally contain other pollutants, such as NO.sub.x, but they do not contain sulfur dioxide.
Increasing significance is being attached both in public debate, and generally in terms of process optimization, to the reduction of emissions of highly toxic halogenated organic compounds, such as in particular PCDDs and PCDFs, as may be contained in the gases arising from chemical and incineration processes. The content of highly toxic PCDDs and PCDFs in such gases may be in the range of 0.1 ng TE/Nm.sup.3 to approximately 30 ng TE/Nm.sup.3 (ng TE/Nm.sup.3 =nanogram toxic equivalent per normal cubic meter). In the 17th implementing regulations of the German Federal emission control law, legislators have provided a limiting value of 0.1 ng TE/Nm.sup.3 beginning in 1996, so improved processes for the removal or degradation of pollutants are consequently required to achieve the legal limiting value.
The highly toxic halogenated organic compounds are in particular PCDDs and PCDFs of varying degrees of chlorination (n and m in the following formula stand for an integer from 1 to 8), which occur in numerous isomers or congeners of varying toxicities: ##STR1##
As is known in this art, the above-mentioned TE value is calculated by multiplying the concentrations of the individual PCDDs and PCDFs by the appropriate NATO-CCMS toxic equivalent factor and by adding the results together. The particularly highly toxic 2,3,7,8-tetrachlorodibenzodioxin has a TE factor of 1. Other TE factors are mentioned by way of example: 0.5 for 1,2,3,7,8-pentaCDD; 0.1 for 1,2,3,4,7,8-hexaCDD; 0.1 for 2,3,7,8-tetraCDF; 0.5 for 2,3,4,7,8-pentaCDF; 0.1 for 1,2,3,4,7,8-hexaCDF.
Previously, those pollutants of the PCDD or PCDF type contained in gases such as flue gases from incineration processes were removed by passing the gases over adsorbents. Adsorption processes using a moving or fixed bed with hearth-oven coke and direct duct injection entrained-bed processes with added lime/activated charcoal are known. Adsorption processes have the disadvantage that a waste gas problem becomes a solid waste problem because the pollutant-laden adsorbent must either be expensively regenerated or disposed of as hazardous waste. Moreover, safety hazards are associated with plants with adsorbers filled with activated charcoal due to CO formation in "hot spots" or even combustion, which makes expensive safety measures necessary.
According to the process disclosed in DE 40 01 305, dioxins and furans may be removed from flue gases by treating the flue gases, after a wet scrubbing stage, with UV radiation of a wavelength of below 310 nm (preferably below 245 nm) in the presence of added hydrogen peroxide. According to the examples in DE 40 01 305, residual dioxin/furan contents of only 1 ng TE/Nm.sup.3 are obtained; this process is clearly not suitable for achieving the residual contents pursuant to the future legal limiting value of 0.1 ng TE/Nm.sup.3. Additionally, there are costly technical difficulties related to the use of the required energy in the form of UV radiation.
The German patent application DE 42 10 055.0 discloses a process for the removal of halogenated compounds such as PCDDs and PCDFs from waste gases optionally containing NO.sub.x, wherein the waste gas is optionally passed through a fixed or fluidized bed and is brought into contact with Caro's acid (H.sub.2 SO.sub.5) or a salt thereof. The Caro's acid may be formed in situ from hydrogen peroxide and sulfuric acid (which is added or is produced from the sulfur dioxide present in the waste gas to be treated and H.sub.2 O.sub.2). Catalytically active solids utilized are inter alia pyrogenic and precipitated silicas, alumina, titanium dioxide and zeolites. DE 42 10 055.0 teaches that Caro's acid plays a decisive part in the oxidation of PCDDs and PCDFs; if the gas to be treated contains no or only a little SO.sub.2, SO.sub.2 or H.sub.2 SO.sub.4 must be added. Although the highly toxic dioxins and furans are over 90% degraded on the solid to less toxic products under the processing conditions, and only a small proportion remains adsorbed on the solid, it must be considered disadvantageous that only gases containing SO.sub.2 are suitable for the process or that SO.sub.2 or H.sub.2 SO.sub.4 must be added.